Naked diazaborepin dyes: Synthesis, photophysical properties, substituent effects and theoretical calculations on ESIPT process

A series of new 3H-Pyrrolizine fused ‘naked’ diazaborepin (NDABs) have been strategically designed and synthesized by replacing one of the hydrogens on pyrrole and/or indole units with electron-donating/withdrawing groups. NDABs contain a seven-membered N-H···N (H-bond) between proton donor (pyrrole) and proton acceptor (indole) moieties. The structures are characterized by 1H NMR, 13C NMR, and high-resolution mass spectrum (HR-MS). Photophysical properties are discussed by the absorption and emission spectra, DFT calculation. NDABs show characteristic strong absorption bands with high molar absorption coefficients, and emission bands with high quantum yields (0.14–0.47) and large Stokes shifts (7270-8025 cm−1). Based on this benchmark, ESIPT process was studied by electronic spectra, geometry, dynamics and thermodynamics analysis. The results demonstrate that the different substitutions at various positions of parent molecule, altered the strength of intramolecular H-bond, tuned the speed of ESIPT process which is consistent with the experimentally observed. On this -NH- type system, ESIPT process has been dramatically affected by the substitution on proton donor moiety and slightly on the proton acceptor moiety.